Dynamic braking of wound motor polyphase induction motors

ABSTRACT

A circuit for effecting dynamic braking of a wound rotor polyphase induction motor comprises rectifier means for rectifying the output of the rotor and switch means which disconnect the stator from the mains supply and connect the rectified rotor output through a d.c. chopper to one or more phase windings of the stator to effect dynamic braking of the motor. The level of direct current in the stator phase windings, and hence the degree of braking, is controlled by the d.c. chopper, which may also be employed to control the driving speed of the motor.

United States Patent Sloan et al.

DYNAMIC BRAKING OF WOUND MOTOR POLYPHASE INDUCTION e e e C ed MOTORSUNITED STATES PATENTS 2 l stair gz fimr ::& 3,504,254 3/1970 Rosenberry,Jr ..31s/237 x England 3,519,912 7/1970 Charlwood et al. ..3l8/2373,529,224 .9/1970 Bedford ..3l8/237 Assignee: Sevcon EngineeringLimited, 3,546,550 12/1970 Badal et al ..3 18/212 Gateshead, Co. Durham,England Primary Examiner-Gene Z. Rubinson Ffled July 1971 Attorney-Rosen& Steinhilper Appl. No.: 159,348 v [57] ABSTRACT Foreign ApplicationPriority Data A circuit for effecting dynamic braking of a wound rotorpolyphase induction motor comprises rectifier a 2 fi means forrectifying the output of the rotor and switch 1970 "17902/70 means whichdisconnect the stator from the mains 1970 Jwltzer an 5/1068 supply andconnect the rectified rotor output through 7 S 59 842 9 a dc chopper toone or more phase windings of the l9 0 -Y 4 stator to effect dynamicbraking of the motor. The Dec. 4, France ..70 level of direct current ithe stator phase d g DCC. 3, Sweden and hence h degree of i g, icontrolled the d.c. chopper, which may also be employed to control (gill..318/21:I,032l82/;; the driving speed of the moton n p Field of Search..318/209-2l2, 237 16 Claims, 5 Drawing Figures BRAKE 62 DRIVE SWITCH/N6SYSTEM F r r RECTIF/EL-SHOOTH/NG PULSE coll/mot l10LAT/N6 STATOR ROTORCONTROL TACHO- 58 omrn- ATOR comr- 56 ATOR MFERENLE SIGNAL PATENTEUHCI 11 I972 SHEET 1 0F 4 is 25% J Q3 QXES mm %2 Y m -523 v -ss \3 v 3 v 5:2233 25s I Q S53 E53! -28 5E 2:32 swam mgm l E8 525% J a 2% l m Q A @v owALBERT E.

ROSEN & STEINHILPER, Attorneys PATENTEDBCT 1. 2' 3.699.416

' SHEET 2 OF 4 ALBERT E. SLOAN ALISTAIR c; TURNB'ULL, Inventors ROSEN &STEINHILPER. Attnrnevs mimmucmmz 3.699.416 sum 3 m4 ALBERT E. SLOANALISTAIR G. TURNBULL, Inventors ROSEN & STEINHILPER, AttorneysPATENTEDHBI 11 m2 saw u [If 4 www mm ALBERTB. SLOAN ALISTAIR G.TURNBULL, Inventors ROSEN & STEINHILPER, Attornevs DYNAMIC BRAKING FWOUND MOTOR POLYPHASE INDUCTION MOTORS supply and to connect therectifier means to a variable mark-space ratio pulse controller and toat least one phase winding of the stator so that the rectified rotoroutput is supplied through the pulse controller to the stator phasewinding thereby to effect dynamic braking of the motor, the pulsecontroller being adapted to control the level of direct current in thestator phase windmg.

Preferably, the switch means are adapted in a second condition thereofto connect the stator phase windings to the mainssupply and to connectthe rectifier means to the pulse controller so that the rectified rotoroutput is supplied to the pulse controller, the pulse controller beingadapted to afford variation of the effective resistance of the rotorthereby to vary the driving speed of the motor.

"Suitably, there is provided a smoothing circuit connected to therectifier means so as to smooth the rectified rotor output and includinga reservoir capacitor so connected to the switch means that on operationof the switch means to effect dynamic braking of the motor the reservoircapacitor begins to discharge through a series path including at leastone of the phase windings of the stator thereby to establish an initialdirect current in the stator phase winding or windings.

Advantageously, the smoothing circuit includes a first unidirectionalcurrent path through which, in operation, the reservoir capacitorreceives charging current from the rectifier means when the pulsecontroller is non-conducting, and a second unidirectional current pathconnected in a series path including the reservoir capacitor and thepulse controller and through which, in use, the reservoir capacitortends to discharge when the pulse controller is conducting, the saidcurrent paths being connected to the switch means in such a manner that,during dynamic braking of the rotor, both the charging and dischargingcurrents of the reservoir capacitor flow in the same direction throughthe stator phase winding or windings.

Suitably, there are provided sensing means for providing a firstelectrical signal dependent on the speed of the rotor, means forproviding a second or reference signal, comparator means for comparingthe magnitudes of the first and second signals and for providing anoutput signal dependent on the magnitudes of the first and secondsignals", the said output signal being supplied to the switch means tocause the switch means to enter its said first condition thereby toeffect dynamic braking of the rotor when the comparison of the first andsecond signals indicatesthat the motor speed is greater than apredetermined value corresponding to the magnitude of the second orreference signal.

The invention will now be described, by way of example, with referenceto the accompanying drawings in which:

FIG. 1 is a block diagram of a control system according to thisinvention, as applied to a three-phase, wound rotor induction motor,

FIG. 2 is a circuit diagram of some of the elements of the controlsystem shown in FIG. 1,

FIGS. 3 and 4 are more detailed diagrams of parts of the circuit of FIG.2, and

FIG. 5 is a circuit diagram of further elements of the control systemshown in FIG. 1.

As shown in FIG. 1, the delta-connected phase windings of the stator 40of a three phase, wound rotor type of induction motor are connected tothe mains supply 44 via mains isolating contactors 42. The wound rotor46 of the motor has its phase windings star connected and across theoutput of the rotor via the appropriate slip rings is a full waverectifier bridge 48 which provide a rectified output. Connected acrossthe rectified output via a smoothing circuit 50 is a variable mark-spaceratio pulse controller 52 which operates, as explained in more detailbelow, to vary the effective resistance of the rotor 46 when the motoris in a driving mode, thereby to vary the speed of the motor, and alsooperates to control the current supplied to the stator 40 from the rotor46 when the motor is in a braking mode as described more fully below.

The speed of rotation of the rotor 46 is measured by a tacho-generator54, which provides an electrical signal proportional to the rotor speed.The signal from the tacho-generator S4 is compared with a referencesignal 56 by means of a comparator 58. If the comparison indicates thatthe speed of the motor is too low, a signal supplied by the comparator58 to the control oscillators 60 governing the pulse controller 52causes the pulse controller to decrease the effective rotor resistanceand thereby effect an increase in the motor speed. If the comparisonindicates that the speed of the motor is too high, a signal from thecomparator 58 to the brake/drive switching system 62 causes the systemto operate the mains isolating contactors 42 to disconnect the stator 40from the mains supply 42, and operates further contactors to connect therectified output of the rotor 44, through the pulse controller 52, totwo of the phase windings of the stator 40 to effect dynamic braking ofthe motor. The current fed to the stator, and hence the dynamic brakingis controlled through the pulse controller 52, as described more fullybelow, in dependence upon the signal received from the comparator 58.

The stator 40, rotor 46, rectifier assembly 48,

smoothing circuit 50, and pulse controller 52 are shown in more detailin FIG. 2.

As shown in FIG. 2, the output of the full-wave recti-' output. Inparallel with the diode 74 and resistor 76 is the series connection of acontactor 78, a diode 80 and U.S. Pat.

Connected to the smoothing circuit is the variable.

mark-space ratio pulse controller 52, comprising a main thyristor 90whose anode is connected, through an inductor 92, to the junction of thecontactor 78and diode 80, and whose cathode is connected to the negativeterminal of the rectifier assembly 48, A commutating capacitor 94,inductor 96 and second thyristor 98 are connec'ted' in series across themain thyristor 90. A conducting path consisting of a resistor 100 anddiode 102 is connected between the junction of the thyristor 82andreservoir capacitor 72 and the junction of the second thyristor 98 andinductor 96, the cathode of diode 102 being connected to the anode ofthyristor 98. An auxiliary supply is provided by connections from aregulated 160 volt supply 116, derived in a suitable manner from themains, through a diode 104 and resistor 106 to the anode of thyristor 98and through a diode 108 and resistor 1 to the anode of thyristor 90.

The gate cathode of the main thyristor 90 is supplied with firing pulsesfrom a control oscillator 112 of wellknown form. The control oscillatorcomprises a relaxation oscillator the period of oscillation of which isgoverned by a transistor the base of which is applied a signal derivedfrom the comparator 58, which is described more fully below. Theoscillator may for example be of the kind shown in British Pat. 950,734,or No. 3,222,582 the above-mentioned transistor taking the place of thevariable resistor controlling the period of the oscillator. Controloscillators of this kind are also illustrated in British Pat. No.963,648 or U.S. Pat. No. 3,337,786.

The gate cathode of the second thyristor 98 is connected to a firingcircuit 118, described more fully below, which supplies a firing pulseto the thyristor 98 when the voltage across the commutating capacitor 94reaches 200 volts. Thyristor 82 receives firing pulses from a firingcircuit 114, described more fully below, the thyristor being fired intoconduction when the voltage across diode 80 and thyristor 82 reaches 150volts.

The junction of two of the phase windings 120 and 122 of the stator isconnected through one pair of contacts of a contactor 126 to thejunction of the contactor 78 and diode 80. The other two junctions ofthe phase windings of the stator are connected through contacts of thecontactor 126 to the junction of-the contactor 78 and the resistor 76. v

The firing circuit 114 associated with thyristor 82 is shown more fullyin FIG. 3. Connected across the diode 80 and thyristor 82 is aresistor-capacitor network comprising a resistor 130 and'a capacitor 132in series, one plate of the capacitor being connected to the cathode ofthyristor 82. A pair of resistors 134 and 136 are connected in parallelwith-the resistor 130 and capacitor 132, and a diode 138 connects thejunction of resistors 134 and .136 to the junction of the resistor 130and capacitor 132. The junction of capacitor 132 and resistor 130 isconnected through a Shockley diode 140 1 to the gate cathode ofthyristor 82. The breakover voltage of Shockley diode 140 is such that afiring pulse is supplied to the thyristor 82 through the Shockley diodewhen the voltage across the capacitor 132 reaches 10 volts. The valuesof resistance of the resistors 134 and 136 are chosen so that capacitor132 is charged rapidly to 10 volts through diode 138 when the voltageacross the diode and thyristor 82 reaches 150 volts. If the voltageacross diode 80 and thyristor 82 is between 10 volts and 150 volts, thecapacitor 132 will charge through diode 138 to the voltage required tocause breakover of the Shockley diode at a rate dependent on the voltageacross diode 80 and thyristor 82. A maximum delay between thecommutation of main thyristor 90 and the firing of thyristor 82 isprovided by resistor 130, through which, in the limit as the voltageacross diode 80 and thyristor 82 approaches 10 volt, capacitor 132 willcharge to the required value after a delay of about twenty milliseconds.A diode 142 is connected between the cathode and the gate cathode ofthyristor 82 to prevent any reverse voltage being applied to the gatecathode, and a gate suppression resistor 144 is connected in parallelwith diode 142.

The firing circuit 118 associated with thyristor 98' is shown moreclearly in FIG. 4. A resistor-capacitor network comprising a resistorand capacitor 152 is connected across the commutating capacitor 94, oneplate of the capacitor 152 being connected to the plate of thecommutating capacitor 94 which is connected to the anode of the mainthyristor 90. A pair of resistors 154 and 156 are connected in parallelwith the resistor 150 and capacitor 152, and a diode 158 connects thejunction of resistors 154 and 156 to the junction of resistor 150 andcapacitor 152. The junction of capacitor 152 and resistor 150 isconnected through a Shockley diode 160 to one end of the primary windingof a transformer 162, the other end of the primary winding beingconnected to the junction of capacitor 152 and resistor 156. Thesecondary winding of transformer 162 is connected across thecathode-gate cathode path of thyristor 98. The values of resistance ofthe resistors 154 and 156 are chosen so that capacitor 152 is chargedrapidly to 10 volts through diode 158 when the voltage across thecommutating capacitor 94 reaches 200 volts. When the voltage acrosscapacitor 152 reaches 10 volts, Shockley diode 160 breaks over and afiring pulse is applied to thyristor 98 through transformer 162. If thevoltage across the commutating capacitor 94 is between 10 and 200 volts,the capacitor 152 will charge through diode 158 to the voltage requiredto cause breakover of the Shockley diode 160 at a rate dependent on thevoltage across the commutating capacitor. A maximum delay for firingthyristor 98 is provided by resistor 150 through which capacitor 152will charge to the requisite voltage, when the voltage across thecommutating capacitor is about l0 volts, after a delay of about 20milliseconds.

The comparator 58 and associated circuitry are shown more fully in FIG.5.

The reference signal 56 is provided by a voltage divider consisting oftwo resistors and 182 connected between a stabilized 40 volt voltagerail 184 and a zero voltage rail' 186. The output of the tacho-generator54 is rectified by means or" a full-wave rectifier (not shown) andapplied to terminals 188 and 190, across which is connected a voltagedivider comprising a pair of resistors 192 and 194. Terminal 188 isconnected also to the junction of resistors 180 and 182, so that thepotential at the junction of resistors 192 and 194 is equal to thedifference between the reference voltage at the junction of resistors180 and 182 and a fixed fraction of the rectified output voltage of thetachogenerator 54.

The junction of resistors 192 and 194 is connected through a diode 196to the base of an n-p-n transistor 198, the collector of which isconnected through a resistor 202 to a stabilized 9 volt voltage rail 200and the emitter of which is connected through a resistor 204 to the zerovoltage rail 186. The emitter of transistor 198 is connected also to thebase of an p-n-p transistor 206, the collector of which is connectedthrough a resistor 208 to the zero voltage rail, and to the emitter of ap-np transistor 210, the collector of which is connected through aresistor 212 to the zero voltage rail 186. The base of transistor 210 isheld at 4.7 volts by means of a zener diode 214 which is connected inseries with a resistor 216 and diode 218 between the 9 volt rail 200 andthe zero voltage rail 186. The emitter of transistor 206 is connected tothe junction of the resistor 216 and the anode of diode 218. The emitterof transistor 206 is thus held at a potential equal to 4.7 volts plusthe voltage drop across diode 218. The collector of transistor 206 isconnected through a resistor 220 and diode 222 to the base of an n-p-ntransistor 224. The collector of transistor 224 is connected through aresistor 226 to a stabilized 18 volt voltage rail 230, and its emitteris connected through a resistor 232 to the zero voltage rail. Thecollector of transistor 210 is similarly connected through a resistor234 and diode 236 to the base of transistor 224. The junction of thebase of transistor 224 with the cathodes of diodes 222 and 236 isconnected through a resistor 238 to the zero voltage rail. The collectorof transistor 224 is connected through line 240 to the controloscillator 112 of the main thyristor 90 and supplies a control voltageto control the frequency of firing of the main thyristor.

The junction of resistor 220 and diode 222 is connected through a diode242 to the collector of an n-p-n transistor 250 of a bistablemultivibrator forming part of the brake-drive switching system 62. Thejunction of resistor 234 and diode 236 is similarly connected through adiode 244 to a second n-p-n transistor 252 of the bistablemultivibrator. The bistable multivibrator is of a well-known form, andconsists of the two n-p-n transistors 250 and 252, the emitters of whichare connected to the zero voltage rail 186 and the collectors of whichare connected through respective resistors 254 and 256 to the 18 voltrail 230. The base of transistor 250 is connected to the collector oftransistor 252 through resistor 258, to the zero voltage rail throughresistor 260, and through a resistor 262 to one plate of a capacitor264, the other plate of which is connected to the rail 186. The base oftransistor 252 is similarly connected through a resistor 266 to thecollector of transistor 250, to the rail 186 through resistor 268, andthrough a resistor 270 to one plate of a capacitor 272, the other plateof which is connected to the rail 186.

The collector of transistor 210 is connected through a resistor 274 tothe junction of capacitor 272 and resistor 270, a diode 276 beingconnected across the resistor 274 with its anode connected to thecapacitor 272. Similarly, the collector of transistor 210 is connectedthrough diode 280 and resistor 282 to the junction of capacitor 264 andresistor 262, a diode 284 being connected across resistor 282 with itsanode connected to capacitor 264.

The junction of the collector of transistor 252 and resistor 254 of thebistable multivibrator is connected through a resistor 286 to the baseof an n-p-n transistor 290, the emitter of which is connected to thezero voltage rail 186. The collector of transistor 290 is connectedthrough line 292 to a relay (not shown) which is energized, whentransistor 290 is in a conducting state, to close the mains isolatingcontactors 42 (see FIG. 2) and to open contactors 126 and 78 to put themotor in a driving condition. When transistor 290 is in a nonconductingstate the relay is de-energized, and the contactors 42, 126 and 78 areoperated to put the motor in a braking condition, as explained below.

A line 300, which can be connected when desired to the 9 volt rail 200,is connected through resistor 302 and diode 304 to the base oftransistor 198, and also through a diode 306 to the junction ofcapacitor 264 and resistor 262 of the bistable multivibrator.

The various stabilized voltage rails are derived from suitable circuitsfed from the mains supply through a full-wave rectifier.

The operation of the various circuits will now be described.

Referring to FIG. 2, when the motor is in its driving mode, the mainsisolating contactors 42 are closed to connect the stator 40 of the motorto the three phase mains supply 44. Braking contactors 126 are open, and

' contactor 78 is closed to connect the main thyristor 90 in series withthe rectifier assembly 48. The rotating magnetic field developed in thestator induces currents in the phase windings of the rotor 46, andcauses the rotor to rotate. The output of the rotor is rectified by therectifier assembly 48 and applied across the main thyristor 90. Thesmoothing circuit 50 prevents high inductive voltages appearing on therotor on turn off of the main thyristor 90. Thus when the main thyristor90 is rendered non-conducting, and thyristor 82 is conducting, chargingcurrent flows into the reservoir capacitor 72 through inductor 70,contactor 78, diode and thyristor 82. When the main thyristor isrendered conducting, the capacitor begins to discharge through diode 74,resistor 76, contactor 78 and thyristor 90. The resistor 84 acrosscontactor 78 permits charging of capacitor 72 before contactor 78 isclosed, to prevent high transient currents when the contactor is closed.

The pulse control circuit operates as follow. Assuming that thyristors98 and 82 are non-conducting, the main thyristor is rendered conductingby a firing pulse from its firing circuit 112. Rotor current thereforeflows through thyristor 90, and reservoir capacitor 72 beings todischarge. Current from reservoir capacitor 72 flows also throughresistor 100, diode 102, and inductor 96 to the commutating capacitor94, which is therefore charged with the plate connected to inductor 96at a positive potential relative to the anode of thyristor 90.

When the voltage across the commutating capacitor 94 reaches 200 volts,or after a maximum delay of twenty milliseconds as described above,thyristor 98 is rendered conducting by firing circuit 118. Thecommutating capacitor 94 begins to discharge through inductor 96 andthyristor 98, so that the main thyristor 90 is reverse biased and istherefore commutated. Capacitor 94 continues to discharge through theinductive circuit comprising inductor 96, thyristor 98, rectifierassembly 48, inductor 70 and contactor 72, and begins to recharge withopposite polarity. Since thyristor 82 is initially non-conducting, therotor current cannot flow into the reservoir capacitor 72, so that rotorcurrent flows intothe commutating capacitor to enhance the charge on it.Thyristor 82 is fired into conduction by firing circuit 114, asdescribed above, when the voltage across the diode 80 and thyristor 82reaches 150 volts. This ensures that the voltage to which thecommutating capacitor 94 is charged exceeds the voltage across thereservoir capacitor 72. When thyristor 82 is rendered conducting, theplate of the commutating capacitor 94 connected to the anode ofthyristor 90 is at approximately the same potential as the plate of thereservoir capacitor 72 which is connected to the cathode of thyristor82. The other plate of the commutating capacitor 94 is therefore at alower potential than the plate of the reservoir capacitor 72 connectedto the cathode of thyristor 98. Thus when the current flowing throughinductor 96 is dissipated, thyristor 98 is reverse biassed and istherefore commutated. When the main thyristor 90 is again renderedconducting, thyristor 82 is reverse biassed and therefore commutated bythe reservoir capacitor 72 which begins to discharge through the mainthyristor 90. The cycle then repeats itself.

It will be appreciated that the provision of thyristor 82 in thecharging path of the reservoir capacitor 72 enables the commutatingcapacitor 94 to be charged to a voltage in excess of that to which itwould be charged if thyristor 82 were omitted or replaced by a diode, toensure commutation of thyristor 98. If thyristor 98 were not commutated,the resistor 100 would be effectively placed across the rectified rotoroutput when the main thyristor was non-conducting, instead of thedesired open circuit.

Similarly, firing thyristor 98 when the voltage across the commutatingcapacitor 94 reaches a predetermined value ensures that the capacitor 94attains a charge sufficient to commutate the main thyristor 90.

The auxiliary supply through line 116 acts to provide a sufficientvoltage across each of the thyristors 90 and 98 to maintain theoperation of the pulse controller when the output voltage of therectifier assembly 48 is very low. I

The provision of a diode 80 and thyristor 82 in series, rather than asingle thyristor, enables a thyristor of relatively low voltage ratingto be used, the combination being less costly than a single thyristorwith the necessary voltage rating.

it will be apparent that, with the motor in its driving mode, the pulsecontrol operates to vary the effective resistance of the rotor. Thuswhen the main thyristor 90 is non-conducting the rotor resistance isinfinite and when the main thyristor 90 is conducting the rotorresistance is effectively zero. Accordingly by varying the rate at whichthe main thyristor 90 is alternately fired into conduction andcommutated, the effective resistance of the rotor, and therefore thespeed of the rotor for any given driving torque, is varied. When thepulse control is acting as a variable resistor in this manner, energy isdissipated by current flowing through resistors 76 and 100.

When dynamic braking of the motor is to be effected, the mains isolatingcontactors 42 are opened to disconnect the phase windings of the stator40 from the mains supply. Contactor 78 is then opened and breakingcontactors 126 are closed. The effect of this is to connect phasewindings 120 and 122 of the stator in parallel and to connect therectifier assembly 48 in series with the phase windings 120 and 122, andthe main thyristor 90. Phase winding 124 of the stator is shortcircuitedthrough contactors 126. Moreover, the reservoir capacitor 72 isconnected,v through diode 74 and resistor 76 in a series circuit withphase windings 120 and 122 of the stator and the main thyristor 90.Thus, when thyristor 90 is rendered conducting, the reservoir capacitor72 begins to discharge through the phase windings 120 and 122. The d.c.current which thus flows in the stator windings establishes a staticflux which is cut by the rotating conductors of the rotor therebycausing a braking torque to be applied to the rotor, and inducingcurrent in the phase windings of the rotor. The induced current isrectified by rectifier assembly 48 and flows through the stator windingsin the series circuit described above, so that the stator current andtherefore the braking torque is increased. The system is therefore apositive feedback system providing dynamic braking of the motor.

It will be evident that the dc. current in the phase windings 120 and122 of the stator 40 can be controlled by varying the mark-space ratioof the pulse control. Thus when the duration of conduction in a fixedtime interval of the main thyristor 90 is a maximum, the braking torquebuilds up most quickly, and as the duration of conduction in a fixedtime interval of the main thyristor 90 decreases so does the rate ofincrease of the braking torque.

The smoothing circuit operates in a similar manner during braking tothat of its operation when the motor is in its driving mode. Thus whenthe main thyristor 90 is commutated, the reservoir capacitor 72 beginsto charge through inductor 70, phase windings 120 and 122 of the stator,diode 80 and thyristor 82. When the main thyristor is fired intoconduction, the reservoir capacitor 72 begins to discharge through theseries path consisting of diode 74, resistor 76, phase windings and 122of the stator, and the main thyristor 90. It will be seen that both thecharging and discharging currents of the reservoir capacitor flow in thesame direction through the stator windings. Thus, in effect, part of therectified rotor current is passed twice through the stator windings, sothat the static flux established in the stator is thereby enhanced.

The operation of the comparator and associated circui'try shown in FIG.5 will now be described.

A voltage signal derived from the reference voltage andtachogenerator-output is applied to the base of transistor 198 asdescribed above. Transistor 198 is connected in an emitter followerconfiguration so that the voltage signal is applied through its emitterto the base of transistor 206 and to the emitter of transistor 210. Ifthe potential of the emitter of transistor 198 is slightly above 4.7volts, both transistors 206 and 210 are held in a non-conductingcondition, and the circuit is in a quiescent state.

If the output voltage of the tachogenerator 54 increases, indicatingthat the rotor speed is too high, the potential of the emitter oftransistor 198 falls below 4.7 volts. Transistor 210 remainsnon-conducting, so that its collector is held at the potential of thezero voltage line 186. Transistor 206 is rendered conducting, so thatcurrent flows through resistor 208 and the collector voltage oftransistor 206 rises towards 4.7 volts. Collector current flows fromtransistor 206 through resistor 274 and charges capacitor 272 of thebistable multivibrator. When capacitor 272 is charged, current flowsthrough resistor 270 to the base of transistor 252 of the multivibrator,turning it on. The potential of the collector of transistor 252therefore falls, as does the potential of the base of transistor 250, sothat transistor 250 is turned off. When transistor 25.2 is thus turnedon, the potential of the base of transistor 290 falls and thistransistor is therefore rendered non-conducting. The relay (not shown)associated with transistor 290 is de-energized, and the contactors 42,126 and 78 are released to put the motor in its braking mode.

When transistor 250 has been turned off, collector current fromtransistor 206 flows also through resistor 220 and diode 222 to the baseof transistor 224, which is therefore rendered conducting. Collectorcurrent therefore flows through resistor 226, and the collector voltageof transistor 224 falls. This voltage, supplied through line 240 to thecontrol oscillator 112 of the main thyristor 90 effects an increase inthe frequency of firing of the thyristor 90, so that dynamic braking ofthe motor is effected until the speed of the motor returns to the speedset by the reference voltage. It will be apparent that when transistor250 is conducting, the anode 222 is held at substantially zeropotential, through diode 242, so that collector current from transistor206 cannot be supplied to the base of transistor 224. Thus the increasein mark-space ratio of the pulse controller cannot be increased untilthe multivibrator has been actuated to select the braking mode of themotor.

If the speed of the motor falls, so that the potential of the emitter oftransistor 198 rises above a potential 4.7

volts plus the voltage drop across diode 218, transistor 210 is renderedconducting, whilst transistor 206 is turned off, its collector voltagefalling to zero. Capacitor 272 of the multivibrator therefore dischargesthrough diode 276, whilst capacitor 264 is fed with charging currentfrom the collector of transistor 210. Transistor 250 is therefore turnedon, and transistor 252 is turned off. The potential of the base oftransistor 296 rises towards the 18 volt line 230, and this transistoris turned on. The relay operating the mains isolating and brakingcontactors is therefore energized, and the motor is switched to itsdriving mode. When transistor 252 is rendered non-conducting, thevoltage at the anodes of diodes 236 and 244 is allowed to rise, andcollector current is supplied through diode 236 to the base oftransistor 224, turning it on. Thus the markspace ratio of the pulsecontrol is increased, and the speed of the motor increases until itreaches its set speed.

it will be evident that the described circuit acts, when the speed ofthe motor departs from its set speed, to

switch the motor to its driving or braking mode as appropriate and thenoperates the pulse control to bring the speed of the motor back to itsset speed.

The circuit also allows a maximum speed to the motor to be selected. Toeffect this, line 300 is connected to the 9 volt rail. A 9 voltpotential is therefore applied through resistor 302 and diode 304 to thebase of transistor 198, over-riding the tachogenerator control andholding transistor 224 conducting to obtain the maximum mark-space ratiofor the pulse control. At the same time, capacitor 264 of themultivibrator is charged rapidly through diode 306, ensuring that themultivibrator is set to select the driving mode for the motor.

It will be apparent that an adjustable reference voltage, and thereforean adjustable predetermined speed of the motor could be obtained byreplacing the resistor network and 182 with an adjustable potentiometer.

We claim:

1. A circuit for effecting dynamic braking of a wound rotor polyphaseinduction motor having stator phase windings fed from a polyphase mainssupply and a wound rotor, comprising rectifier means connected to therotor so as to rectify the output of the rotor, a variable mark-spaceratio pulse controller, switch means comprising first switchingcontactor means connected between the polyphase mains supply and thestator phase windings of the motor and operable in a first condition ofthe switch means to disconnect the stator phase windings from the mainssupply, second switching contactor means connected between the rectifiermeans and the pulse controller and third switching contactor meansconnected between said pulse controller and said stator phase windings,said second and third contactor means being operable in said firstcondition of the switch means to connect the output of said rectifiermeans through said pulse controller to said stator phase windings,thereby to effect dynamic braking of the motor, the pulse controllerbeing operable to control the level of direct current in said statorphase windings during such braking.

2. A circuit as claimed in claim 1, wherein in a second condition of theswitch means the first contactor means are operable to connect saidstator phase windings to said mains supply and the second contactormeans are operable to connect said pulse conti'oller across saidrectified rotor output, whereby the pulse controller varies the rotorcurrent and hence the effective resistance of the rotor thereby to varythe driving speed of the motor.

3. A circuit as claimed in claim 2, wherein there is provided asmoothing circuit connected to the rectifier means so as to smooth therectified rotor output and comprising a reservoir capacitor which, whenthe switch means is in its first condition, is connected in a seriespath including said pulse controller and at least one of said statorphase windings, whereby, on operation of the switch means from itssecond to its first condition, the reservoir capacitor begins todischarge through said stator phase winding or windings thereby toestablish an initial direct current through said stator phase winding orwindings.

4. A circuit as claimed in claim 3, in which the smoothing circuitincludes a first unidirectional current path connected across therectifier means and the reservoir capacitor receives charging currentfrom the rectifier means when the pulse controller is non-conducting,and a second unidirectional current path connected in a series pathincluding the reservoir capacitor and the pulse controller and throughwhich the reservoir capacitor tends to discharge when the pulsecontroller is conducting, the said current paths being connected to theswitch means in such amanner that, during dynamic braking of the rotor,both the charging and discharging currents of the reservoir capacitorflow in the same direction through the stator phase winding or windings.

5. A circuit as claimed in claim 4, in which the pulse controllercomprises a main thyristor connected to the rectifier means in parallelwith the reservoir capacitor and said first and second unidirectionalcurrent paths, a commutating capacitor connected, in series with aninductor and a second thyristor, across the main thyristor, and meansfor charging the commutating capacitor in such a direction that onfiring the second thyristor into conduction the commutating capacitorreverse biases and so commutates the main thyristor, the commutatingcapacitor then being charged in the reverse direction from the rectifiedrotor output through the series path including the inductor and secondthyristor and when fully charged reverse biasing the second thyristor tocommutate it, and in which the first unidirectional current path of thesmoothing circuit includes a semiconductor switching device adapted tobe rendered conducting when the reverse charge on the commutatingcapacitor has reached a predetermined value sufficient to ensure thatthe second thyristor is commutated when the commutating capacitor isfully charged.

6. A circuit as claimed in claim 1 and adapted for use with athree-phase induction motor the stator phase windings of which aredelta-connected, in which the switch means are adapted to connect two ofthe stator phase windings in parallel to the rectified rotor output andto short circuit the remaining phase winding.

7. A circuit as claimed in claim 1 in which there are provided sensingmeans for providing a first electrical signal dependent on the speed ofthe rotor, means for providing a first electrical signal dependent onthe speed of the rotor, means for providing a second or referencesignal, comparator means for comparing the magnitudes of the first andsecond signals and for providing an output signal dependent on themagnitudes of the first and second signals, the said output signal beingsupplied to the switch means to cause the switch means to enter its saidfirst condition thereby to effect dynamic braking of the rotor when thecomparison of the first and second signals indicates that the motorspeed is greater than a predetermined value corresponding to themagnitude of the second or reference signal.

8. A circuit as claimed in claim 7, in which the comparator means isconnected to the variable mark-space ratio pulse controller so as tovary, whenthe switch means is in its first condition, the mark-spaceratio of the pulse controller in dependence on the magnitude of theoutput signal of the comparator means thereby to control the level ofdirect current in the stator phase winding.

9. Acircuit as claimed in claim 8, in which the switch means are adaptedin a second condition thereof to connect the stator phase windings tothe mains supply and to connect the rectifier means to the pulsecontroller so that the rectified rotor output is supplied to the pulsecontroller, the pulse controller being adapted to afford variation ofthe effective resistance of the rotor thereby to vary the driving speedof the motor, and in which the comparator means are connected to thepulse controller so as to vary, when the switch means is in its secondcondition, the mark-space ratio of the pulse controller in dependence ontheoutput signal of the comparator means thereby to control the drivingspeed of the motor.

10. A circuit as claimed in claim 7, in which manually operated controlmeans are provided for varying the second or reference signal thereby tovary the said predetermined speed of the motor.

11. A control circuit for speed control of polyphase wound rotorinduction motors comprising rectifier means for rectifying theelectrical output of the rotor, sensing means for providing a firstelectrical signal dependent upon the velocity of the rotor, means forproviding a second or reference signal, comparator means for comparingand providing an output signal dependent on the magnitudes of the firstand second signals, a variable mark-space ratio pulse controllerconnected to the rectifier means and adapted upon variation of the markspace ratio thereof to vary the resistance of the rotor circuit, meansfor connecting the comparator output to the pulse controller thereby tovary the mark-space ratio of the pulse controller and switching meansactuated, when the difference between the first and second signal is ina reverse sense with respect to that which causes the rotor to drive, toconnect the rectified rotor output to the stator and disconnect thestator from the electrical supply thereby to effect dynamic braking ofthe rotor.

12. Control apparatus for controlling the speed of a wound rotorpolyphase induction motor, comprising means for rectifying the output ofthe rotor, a variable mark-space ratio pulse controller comprising amain thyristor connected across the rectified rotor output, acommutating capacitor connected, in series with an inductor and a secondthyristor, across the main thyristor, and means for charging thecommutating capacitor from the rectified rotor output in such adirection that on firing the second thyristor into conduction thecommutating capacitor reverse biases and so commutates the mainthyristor, the commutating capacitor then being charged in the reversedirection through the inductive series path including the rectifiedrotor output and the second thyristor and when fully charged reversebiasing the second thyristor to commutate it, the apparatus furtherincluding a smoothing circuit comprising a reservoir capacitor arrangedto receive charging current from the rectified rotor output through afirst unidirectional current path when the main thyristor is commutatedand to discharge through a second unidirectional current path and themain thyristor when the main thyristor is rendered conducting, the firstunidirectional current path including a semiconductor switching device,and firing means operable to render conducting the semiconductorswitching device when the charge on the commutating capacitor in thesaid reverse direction has reached a predetermined value sufficient toensure that the second thyristor is commutated when the commutatingcapacitor is fully charged.

13. Control apparatus as claimed in claim 12, wherein the semiconductorswitching device is a thyristor.

14. Control apparatus as claimed in claim 12, wherein the means forcharging the commutating capacitor from the rectified rotor outputcomprises a unidirectional current path connecting the junction of thereservoir capacitor and the said first and second unidirectional currentpaths of the smoothing circuit to the series circuit including thecommutating capacitor, inductor and second thyristor.

15. Control apparatus as claimed in claim '12, wherein the firing meansfor the semiconductor switching device is operable to render conductingsaid switching device a predetermined time after the main thyristor iscommutated if the reverse charge on the commutating capacitor has notreached said predetermined value within said predetermined time.

16. Control apparatus as claimed in claim 12, wherein there is provideda firing circuit for said second thyristor, said firing circuit beingconnected to the commutating capacitor and operable to supply a firingpulse to said second thyristor to render it conducting when the forwardcharge on said commutating capacitor reaches a predetermined value.

1. A circuit for effecting dynamic braking of a wound rotor polyphaseinduction motor having stator phase windings fed from a polyphase mainssupply and a wound rotor, comprising rectifier means connected to therotor so as to rectify the output of the rotor, a variable mark-spaceratio pulse controller, switch means comprising first switchingcontactor means connected between the polyphase mains supply and thestator phase windings of the motor and operable in a first condition ofthe switch means to disconnect the stator phase windings from the mainssupply, second switching contactor means connected between the rectifiermeans and the pulse controller and third switching contactor meansconnected between said pulse controller and said stator phase windings,said second and third contactor means being operable in said firstcondition of the switch means to connect the output of said rectifiermeans through said pulse controller to said stator phase windings,thereby to effect dynamic braking of the motor, the pulse controllerbeing operable to control the level of direct current in said statorphase windings during such braking.
 2. A circuit as claimed in claim 1,wherein in a second condition of the switch means the first contactormeans are operable to connect said stator phase windings to said mainssupply and the second contactor means are operable to connect said pulsecontroller across said rectified rotor output, whereby the pulsecontroller varies the rotor current and hence the effective resistanceof the rotor thereby to vary the driving speed of the motor.
 3. Acircuit as claimed in claim 2, wherein there is provided a smoothingcircuit connected to the rectifier means so as to smooth the rectifiedrotor output and comprising a reservoir capacitor which, when the switchmeans is in its first condition, is connected in a series path includingsaid pulse controller and at least one of said stator phase windings,whereby, on operation of the switch means from its second to its firstcondition, the reservoir capacitor begins to discharge through saidstator phase winding or windings thereby to establish an initial directcurrent through said stator phase winding or windings.
 4. A circuit asclaimed in claim 3, in which the smoothing circuit includes a firstunidirectional current path connected across the rectifier means and thereservoir capacitor receives charging current from the rectifier meanswhen the pulse controller is non-conducting, and a second unidirectionalcurrent path connected in a series path including the reservoircapacitor and the pulse controller and through which the reservoircapacitor tends to discharge when the pulse controller is conducting,the said current paths being connected to the switch means in such amanner that, during dynamic braking of the rotor, both the charging anddischarging currents of the reservoir capacitor flow in the samedirection through the stator phase winDing or windings.
 5. A circuit asclaimed in claim 4, in which the pulse controller comprises a mainthyristor connected to the rectifier means in parallel with thereservoir capacitor and said first and second unidirectional currentpaths, a commutating capacitor connected, in series with an inductor anda second thyristor, across the main thyristor, and means for chargingthe commutating capacitor in such a direction that on firing the secondthyristor into conduction the commutating capacitor reverse biases andso commutates the main thyristor, the commutating capacitor then beingcharged in the reverse direction from the rectified rotor output throughthe series path including the inductor and second thyristor and whenfully charged reverse biasing the second thyristor to commutate it, andin which the first unidirectional current path of the smoothing circuitincludes a semiconductor switching device adapted to be renderedconducting when the reverse charge on the commutating capacitor hasreached a predetermined value sufficient to ensure that the secondthyristor is commutated when the commutating capacitor is fully charged.6. A circuit as claimed in claim 1 and adapted for use with athree-phase induction motor the stator phase windings of which aredelta-connected, in which the switch means are adapted to connect two ofthe stator phase windings in parallel to the rectified rotor output andto short circuit the remaining phase winding.
 7. A circuit as claimed inclaim 1 in which there are provided sensing means for providing a firstelectrical signal dependent on the speed of the rotor, means forproviding a first electrical signal dependent on the speed of the rotor,means for providing a second or reference signal, comparator means forcomparing the magnitudes of the first and second signals and forproviding an output signal dependent on the magnitudes of the first andsecond signals, the said output signal being supplied to the switchmeans to cause the switch means to enter its said first conditionthereby to effect dynamic braking of the rotor when the comparison ofthe first and second signals indicates that the motor speed is greaterthan a predetermined value corresponding to the magnitude of the secondor reference signal.
 8. A circuit as claimed in claim 7, in which thecomparator means is connected to the variable mark-space ratio pulsecontroller so as to vary, when the switch means is in its firstcondition, the mark-space ratio of the pulse controller in dependence onthe magnitude of the output signal of the comparator means thereby tocontrol the level of direct current in the stator phase winding.
 9. Acircuit as claimed in claim 8, in which the switch means are adapted ina second condition thereof to connect the stator phase windings to themains supply and to connect the rectifier means to the pulse controllerso that the rectified rotor output is supplied to the pulse controller,the pulse controller being adapted to afford variation of the effectiveresistance of the rotor thereby to vary the driving speed of the motor,and in which the comparator means are connected to the pulse controllerso as to vary, when the switch means is in its second condition, themark-space ratio of the pulse controller in dependence on the outputsignal of the comparator means thereby to control the driving speed ofthe motor.
 10. A circuit as claimed in claim 7, in which manuallyoperated control means are provided for varying the second or referencesignal thereby to vary the said predetermined speed of the motor.
 11. Acontrol circuit for speed control of polyphase wound rotor inductionmotors comprising rectifier means for rectifying the electrical outputof the rotor, sensing means for providing a first electrical signaldependent upon the velocity of the rotor, means for providing a secondor reference signal, comparator means for comparing and providing anoutput signal dependent on the magnitudes of the first and secondsignals, a variable mark-space ratio pulse controller connected to therectifier means and adapted upon variation of the mark space ratiothereof to vary the resistance of the rotor circuit, means forconnecting the comparator output to the pulse controller thereby to varythe mark-space ratio of the pulse controller and switching meansactuated, when the difference between the first and second signal is ina reverse sense with respect to that which causes the rotor to drive, toconnect the rectified rotor output to the stator and disconnect thestator from the electrical supply thereby to effect dynamic braking ofthe rotor.
 12. Control apparatus for controlling the speed of a woundrotor polyphase induction motor, comprising means for rectifying theoutput of the rotor, a variable mark-space ratio pulse controllercomprising a main thyristor connected across the rectified rotor output,a commutating capacitor connected, in series with an inductor and asecond thyristor, across the main thyristor, and means for charging thecommutating capacitor from the rectified rotor output in such adirection that on firing the second thyristor into conduction thecommutating capacitor reverse biases and so commutates the mainthyristor, the commutating capacitor then being charged in the reversedirection through the inductive series path including the rectifiedrotor output and the second thyristor and when fully charged reversebiasing the second thyristor to commutate it, the apparatus furtherincluding a smoothing circuit comprising a reservoir capacitor arrangedto receive charging current from the rectified rotor output through afirst unidirectional current path when the main thyristor is commutatedand to discharge through a second unidirectional current path and themain thyristor when the main thyristor is rendered conducting, the firstunidirectional current path including a semiconductor switching device,and firing means operable to render conducting the semiconductorswitching device when the charge on the commutating capacitor in thesaid reverse direction has reached a predetermined value sufficient toensure that the second thyristor is commutated when the commutatingcapacitor is fully charged.
 13. Control apparatus as claimed in claim12, wherein the semiconductor switching device is a thyristor. 14.Control apparatus as claimed in claim 12, wherein the means for chargingthe commutating capacitor from the rectified rotor output comprises aunidirectional current path connecting the junction of the reservoircapacitor and the said first and second unidirectional current paths ofthe smoothing circuit to the series circuit including the commutatingcapacitor, inductor and second thyristor.
 15. Control apparatus asclaimed in claim 12, wherein the firing means for the semiconductorswitching device is operable to render conducting said switching devicea predetermined time after the main thyristor is commutated if thereverse charge on the commutating capacitor has not reached saidpredetermined value within said predetermined time.
 16. Controlapparatus as claimed in claim 12, wherein there is provided a firingcircuit for said second thyristor, said firing circuit being connectedto the commutating capacitor and operable to supply a firing pulse tosaid second thyristor to render it conducting when the forward charge onsaid commutating capacitor reaches a predetermined value.